It is often said that the vice president serves two purposes: 1) to balance the ticket, 2) DO NO HARM! (to win more votes than he or she would lose).

What does Obama need for balance? Region, age, experience, race and party unity could all come in to play. A lot of parallels have been drawn between Obama's campaign and that of Bill Clinton in 1992. In that case, Clinton largely doubled down on the balance question by presenting another young southern centrist, though one with more federal and foreign experience. Obama might chose another young reformer, to reinforce his message of changing Washington while running against a man who has worked on Capitol Hill since the late 1970s. On the other hand, when running against a well experienced, older war hero, he might opt for a more traditional balance choice in chosing someone older, more experienced who is either a war hero or who has military credentials.

I'll lay out all of the candidates I've been able to find listed, and add some of my own, and give a brief run down on my thoughts. I'll then give you a final ranked list of who I think is most likely.

Without further ado, in alphabetical order:

Sen. Evan Bayh (D-IN)Bayh is an attractive candidate having served for eight years as governor of Indiana, a traditionally Republican state which some have argued could be in play under the right circumstances, and as its senator since 1999. After briefly considering his own run for the White House, he endorsed and has been a very strong supporter of Hillary Clinton and thus could bring unity to the ticket. He has served on both the Armed Forces and Intelligence committees in the Senate giving him national security credentials, however he has never served in the military.

On the other hand, Bayh is noted as being not very charismatic and has likely tarnished his creditials in terms of being able to swing Indiana in favour of the top of the ticket after Clinton barely won his state despite demographic favourability and momentum coming out of Pennsylvania.

Sen. Joe Biden (D-DE)I am a big fan of Joe Biden. I think he would have been an excellent presidential candidate this year but his campaign was DOA. Those who argue Biden would make a good VP candidate for Obama cite his many years of experience. He was elected to the Senate in 1972 before turning the required-to-serve-age of 30. He was considered to be almost certainly the Secretary of State had John Kerry won in 2004. He has served as chair of the Senate Judiciary Committee and currenly serves as chair of the Foreign Relations Committee. His plan to bring peace to Iraq has been lauded and was endorsed by a bipartisan super majority of the Senate. Some have even suggested, sort of sarcastically, that Obama needs a racist as a running mate and that Biden fits that bill from his comments about Obama when Biden entered the presidential race.

However, I don't think Biden is the right guy for Obama. Though he needs someone with experience, he needs it in the form of someone who can also be credibly billed as a "Washington outsider". Obama's whole campaign, particularly when postured against McCain, is that McCain has served in the Senate too long and is out of touch with America. Biden has served almost twice as long as McCain, so his candidacy would make no sense.

Mayor Mike Bloomberg (I-NY)For all of the opposite reasons that Biden doesn't make sense, Bloomberg doesn't make sense either. Obama is going to be painted by the Republicans as an out-of-touch liberal. Bloomberg makes the cariacture complete. Besides, I can't imagine he would be interested in being vice president anyway.

Fmr. Sen. Bill Bradley (D-NJ)Bradley makes a lot of sense on paper. He has many years of experience in Washington, yet is viewed as an outsider. His chances however would be somewhat damaged due to his left-wing campaign for president in 2000 in which he abandoned some of his previously moderate positions. The fact that he has endorsed Obama and opposed the establishment candidate Gore in 2000 would make it difficult for him to help unify with the Clinton wing of the party.

However, he would have an outside chance of being nominated as a candidate who meshes with Obama, has an outsider image and has experience.

Sen. Bob Casey, Jr. (D-PA)This name has been floated, as near as I can tell, only because Casey was the senior Pennsylvanian to support Obama. However, he has been in the Senate for barely a year and is pro-life. I can't imagine him being picked.

Fmr. Sen. Lincoln Chafee (I-RI)A former liberal Republican senator from New England who, after being defeated for being a Republican, became independent and endorsed Obama's presidential candidacy could actually be an interesting choice for Obama. One of the biggest challenges that Obama faces will be McCain's ability to attract Rockefeller Republicans who have voted Democratic in recent elections, particularly in states like Maine, New Hampshire, Connecticut and New Jersey which I believe McCain puts in play. Chafee, unlike McCain, is a real liberal Republican and could neutralize that effect. And, indeed, Chafee is a strong liberal (by American standards) like Obama so they would mesh well on policy issues. However, due to his having been a Republican senator until 2006, he would be prevented from being painted as too far left. This could be a real darkhorse choice for Obama.

Ret. Gen. Wes Clark (D-AR)Notwithstanding his poor showing in the 2004 Democratic primaries, Wes Clark is likely a frontrunner, or at least should be, for the veep slot on the Obama ticket. Clark has oodles of military experience to bring to the ticket without Washington baggage and has been a strong Clinton supporter who could help unify the party. Moreover, a southern White former general goes a long was to neutralize John McCain's military credentials and the fears of southern White traditional Democrats who are reluctant to support Obama.

Sen. Hillary Clinton (D-NY)This makes so little sense and so much sense all at the same time, that Politico.com actually posted a piece by the same authors arguing bothsides of the issue.

I don't think that Clinton would be legitimately interested in this route. And her long standing divisive role in Washington politics takes away from Obama's central message. Obama would be better served to nominate an outsider like Clark who supported Clinton but doesn't have her baggage.

That said however, Obama will face a lot of pressure to offer the slot to Clinton and Clinton, if she truly believes Obama is going to lose either way, would be smart to run with him so as to innoculate herself against claims that she tried to sabotage his campaign and thus enable her to get the nomination in 2012.

Fmr. Sen. Tom Daschle (D-SD)Back in January, I floated his name for this gig. However, the more I think about it, the less likely I think it is. Daschle was the ultimate insider (Senate majority leader), yet had little play on military and foreign policy issues while in the Senate. He has been a long time Obama supporter and thus cannot unify the party. The Republicans have ample anti-Daschle research done and proved effective from their successful campaign to defeat him in 2004. Not a wise pick.

Fmr. Sen. John Edwards (D-NC)John Edwards would be a good choice. He appeals to the same rural white voters that Clinton has been beating Obama for, but much more so. In fact, had Edwards not dropped when he did, I wonder if he might be running second now and Clinton third. Edwards proved a very able candidate both in the nomination in 2004 and 2008 and as the vice-presidential candidate in 2004. John Kerry supporters lament the fact that he was not loyal enough after Kerry threw away the 2004 campaign but, afterall, Kerry threw away the 2004 campaign. [Much of this post, including this blurb on Edwards has been in the works for a while. I wrote about Edwards before he endorsed Obama. I think that that endorsement makes Edwards a less likely veep pick because he alienated Clinton supporters by trying to strong-arm her out of the race with his strategically timed endorsement of Obama. He would however remain a very strong choice for the reasons stated here.]

Fmr. VP Al Gore (D-TN)The fact that this totally unlikely scenario gets any ink makes me laugh. Gore might top a ticket with Obama as veep if there was a brokered convention (which there won't be) but he certainly won't run for a third vice-presidential term. Period.

Sen. Chuck Hagel (R-NE)Barack Obama is against the Iraq War. Chuck Hagel is against the Iraq War. On every single other issue imagable, the liberal Democrat and conservative Republican strongly disagree. This idea has little basis in reality and, like Gore, ink should not be wasted on the proposition. Hagel is a likely pick however for an Obama cabinet were he to win the presidency.

Fmr. Rep. Lee Hamilton (D-IN)Lee Hamilton, as former vice-chair of the commission on the terrorist attacks of September 11, 2001, certainly has strong national security credentials. However he is old, frail looking and without charisma. Obama could get as good (or nearly as good) credentials by looking elsewhere.

Gov. Tim Kaine (D-VA)Tim Kaine may be the only person on this list with less-to-equal experience compared to Barack Obama. Kaine has four years as lieutentant governor and about two as governor. Obama has six years as state senator and about three as federal senator. Kaine would certainly reinforce the outsider image presented by Obama but it would also reinforce his image as lacking depth on national security and so forth. Moreover, it would hand the governorship of Democratic-trending Virginia over to the Republican lieutentant governor.

Sen. Blanche Lincoln (D-AR)Senator Lincoln is from Arkansas and has strong appeal to rural, southern Whites. She is female and a supporter of Hillary Clinton from the former Clinton home state. These factors make her a possibility, but her lack of foreign policy experience does not help Obama balance the ticket.

Gov. Janet Napolitano (D-AZ)No national security experience. No charisma. Villain of the right for having represented the infamous woman who accused Clarence Thomas of being sexist during his Supreme Court confirmation hearings. I can't imagine she would be being considered if she weren't a female governor.

Fmr. Sen. Sam Nunn (D-GA)Nunn mused about running for president himself if a "post-partisan" candidate didn't emerge. Nunn and his co-conspirators (Bloomberg, David Boren, etc) now seem to have settled on Obama as their candidate. This would seem to make Nunn a good fit. Add to this the fact that Nunn has impeccible national security credentials and has been gone from Washington long enough to make him a plausible outsider and he starts to seem a very good fit. Moreover, if Obama is serious about making a play in Georgia and the Carolinas, this is the man to help him.

Sen. Harry Reid (D-NV)The only time I saw this floated was in reference to an odd plan in which Hillary was convinced to drop out in exchange for the Senate leadership and Reid was convinced to give up the Senate leadership in exchange for the vice presidential nomination. That doesn't make a lot of sense. However, Reid is a compelling candidate for other reasons. He is a popular moderate from the Mountain West which Obama wants to put in play. Carrying Nevada, Colorado and New Mexico is key to an Obama victory plan.

Gov. Bill Richardson (D-NM)Richardson on the surface seems a compelling candidate. A former presidential contender himself with virtually unending experience in all things government who has (or had before endorsing Obama) close ties to the Clintons. Another plus, he is the governor of a swing state that Obama probably needs to win and a strong leader among Latinos where Obama needs to do some work. However, Richardson has a lot of baggage including lying for decades about being drafted to MLB and allegations of sexual harrassment.

Fmr. Rep. Tim Roemer (D-IN)Roemer served in Washington for a number of years but was never considered an insider. He has considerable national security experience having been the ranking Democrat on the House Intelligence Committee and a member of the commission on the terrorist attacks of September 11, 2001. Roemer would not be necessarily very palatable to Democrats due to his pro-life and general moderate-to-conservative stances on a number of issues.

Gov. Kathleen Sebelius (D-KS)Unlike the other female governor on this list, Sebelius has a real shot at being on Obama's ticket. Sebelius is a rising star in the Democratic Party and has been twice elected governor of Kansas despite being a liberal-to-moderate on issues such as gay marriage, abortion and capital punishment. She would bring strong executive experience to the ticket without diminishing its credential as being "outside the beltway". She also brings advantages to the ticket being a native Ohioan (her father was in fact that governor of that state) and having summered her whole life in Michigan - two large states that Obama needs to carry. Though it would be unlikely she could put Kansas in play, she could be helpful in critical states like Iowa and Missouri. Finally, she is a Catholic and Obama has struggled with that key demographic in the primaries.

The only drawbacks to a Sebelius candidacy would be her lack of foreign policy experience, which the ticket needs, and superficially her awkward last name.

Gov. Ted Strickland (D-OH)Strickland is the governor of a key swing state and was a big Clinton supporter. These are the only real advantages I can see him bringing to the ticket. He has been governor for barely a year and prior to that was in Congress where he didn't have any roles relating to national security.

Gov. Brian Schweitzer (D-MT)Though Schweitzer is very charismatic and comes from a state carried by Bill Clinton in 1992, he is not a very attractive candidate for vice president. He is a conservative Democrat who would not be embraced by the convention and Montana will not be in play - Clinton only carried it due to high numbers of votes received by Ross Perot at the expense of George H. W. Bush.

Sen. Jim Webb (D-VA)Though Webb was just elected to the Senate in 2006, he has lots of experience to make him an attractive candidate for Obama. He is a veteran and served in the Reagan administration in senior defence roles before switching from Republican to Democrat in recent years. He is an authentic and popular southerner. However, he does not do a lot for party unity.

Ret. Gen. Anthony Zinni (D-VA or PA)Zinni has no real political experience and Wes Clark could bring as much to the table in a more effective manner.